A few readers shared that they look forward to the outcome
of my Jupiter direct conversion receiver experiments – so do I! At this point, I haven’t nailed it down and
go/went/reach in all directions like a drunken sailor.I’ll show a few of my primitive style radio
design experiments and often feel stuck in a groove with my mostly 1980’s-90s
circuitry.This blog flaunts neither
Arduino stuff, nor C++ objects coded to perfection; just really ugly analog circuits
and some measures. Analog radio plus binary thinking still works for me.

Inspiration

I love the works of Anton Chekhov. Physician, scientist, and
— a staggeringly good short story author — who penned crisp, slice-of-live
narratives that both entertain + prod you to think deeply. Chekhov inspires my
aude sapere — “my dare to know” self and supplies the tone for my blog: a thin slice
of 1 particular QRP workbench.

This post
arose because I wanted to design my own Jupiter receiver, sought fresh circuitry — and
in the end, wasted a ton of time and parts just to learn a few new things. But that’s
what I do.

Every builder, I suppose, learns in their own way — I believe
that making and measuring real circuit’s yields dividends par none.Hats off to the SPICE kings — simulation
might be the next best thing for those who lack the instruments needed to measure
data and make comparisons — and for getting starting values for an experiment.To me, however — real versus simulated life including
electronics just feels better.

So I set out to make a frequency agile oscillator and
struggled, sputtered and got beat up in the process.Old Murphy, stupid mistakes, bad parts and
the-like tangled up nearly every LO experiment. As a result I didn't build the best possible LO due to fatigue and frustration — but at least I felt reinvigorated to explore
mixers.

My LO strategy involved mixing a 16.93 MHz xtal oscillator ( xtal Q = ~100K ) with a 3.121 to 3.216 MHz Hartley L-C VFO. I planned to use the EMRFD Figure 4.24 method to extract low noise + distortion from the xtal oscillator and mix it with the VFO signal in a Gilbert cell mixer like the NE612.

A reader sent me 4 NE612s last year — it turns out all of them were fried. Sadly, I didn't suspect these mixers until much in-situ debugging ---- wasting parts + time. I didn't want to wait until fresh NE612 mixers arrived and set out to make a homebrew mixer on the bench.

First I'll show the L-C VFO:

Above — VFO schematic with a buffer giving a 50 Ω output impedance The output power measured -8.7 to -10 dBm across its tuning range. With practice, it's fairly easy to make a temperature stable VFO in the 1-3.5 MHz range. I employed light resonator coupling and perhaps overkill DC filtering + output buffering. While testing, I could not pull the VFO frequency with downstream manipulation despite trying hard to do so.

Above — Another super buffer schematic with measures. In Winter 2015 I explored ways to cascade common base amps to develop "super buffers" possessing really high reverse isolation. L-network matching tends to work best @ 1 frequency, however, with experimentation, they can work over a limited range with respect to VFO output amplitude flatness.

Above — VFO schematic built on single sided Cu board. The resonator T50-6 toroid is both zap strapped and epoxy glued to a piece of copper-clad board. A red colored wire provides the 1 link output to the VFO buffer.

Mixer Experiments

I built a few active mixers and will refer to just 2:

Time to add some balance:

Above — A mixer with differential RF and IF ports. Some tuning on the output serves to further suppress the LO and RF at the IF port. I applied discrete, matched transistors, but sometimes build them on a CA3046 BJT array for even better transistor matching.

Above — Analysis of the BJT mixer output. The LO and RF are roughly down the same power from the desired sum IF @ ~20 MHz. (LO and RF = 36-38 dB down). From my real-world balanced mixer experiments, you've achieved reasonable balance when the LO and RF are >= 30 dB down from the LO & RF sum + difference frequencies.

Although usable, I realized I wouldn't have enough room to build all the needed filtration plus amplification circuitry and thus went to a tiny MCL TUF-1 diode ring mixer.

To run this diode ring mixer, I changed the buffer on the L-C VFO to that already shown and adapted the ever-evolving 16.91 MHz xtal oscillator circuit to chop the TUF-1 LO port with a solid 50 Ω termination @ ~ 7dBm.

I still needed to filter and amplify the TUF-1 mixer output to make a usable 20.1 MHz local oscillator for my Jupiter receiver. I didn't have a lot of board space left and had to compromise.

Above — Post mixer filter amplifier sandwich. I've never tried this before in a post-mixer filter scheme and ran T68-6 toroids [ should have run size 50 or less 'roids to save space ]. At some point I must have shorted and fried the MPSH81 and had to clip it out of the crammed board and replace it. This oscillator project fought me from start to finish.

Frequency domain analysis of the entire 20.1 MHz local oscillator output. All non-20.1 MHz tones are down by at least 54 dB. Not great for me, but OK. Prior to placing the board in its metal box, the LO and RF tones were down closer to 60 dB, but the metal box de-tuned, and/or de-Q'ed my T68-6 filter resonators.

Lacking both time and patience to spend more time wrestling with this oscillator. I just bolted the lid on and sighed in relief when it was done.

The 16.93 MHz LO tone is especially tough to filter since it's close to the IF. A 16.93 MHz notch filter might be employed downstream from the oscillator box. The output power = -6.79 dBm.

Above — The final box. A lot of work and parts for a simple 20.1 MHz VFO! It might be better to make a synthesizer with a Si5351A or a VXO affair, but I went my own long way.

Triple-Tuned Filter Preamplifier

I'll show the current favorite Jupiter receiver preamplifier circuit. I'm not sure if I'll keep it, but its transfer function and gain look fabulous.

Above — The schematic of the input filter with PNP common gate amplifier.

I'm still experimenting to find a good mixer (product detector) for my Jupiter receiver.

Here's a couple of candidates I've tested so far:

Above — A JFET bridge variant like this might be seen in high IIP3 professional grade receivers.

Above — SA plot with the RF @ 10 MHz and LO @ 25 MHz. As you turn the 10K balance pots, you'll see the RF and LO tones change in power. I tweaked the 2 pots to get the best balance (the lowest LO and RF power). Spectrum analyzers = serious fun. After getting the best possible balance, I then performed the mixer's 2 tone testing.

I plan to investigate the Trask (N7ZWY) KISS mixer. Here's my first experiment.

Above — I lacked many of the specified parts such as the Fairchild FSA3157 and those expensive MCL transformers, but made do with a dual FET bus switch, the CBT3306.

The LO transistor switch + D flip-flop come right out of EMRFD.

I recently ordered some parts to asses more KISS mixer variants including the FSA3157 and also stocked up on some obsolete, but very fast 74F series logic including the 74F04 and 74F74 that will clock up to 125 MHz or so.

Above — The FET bus switch evaluated as a mixer. The LO and RF are ~ 35 dB down @ the IF port. You can see tones at ~ 12 and 14 MHz — I've got to get used to working with digital circuitry in my RF mixers.

Above 3 — KISS mixer breadboard photo. Again, to provide proper RF ground for all output frequencies, I employ double-sided copper board with copper via wires around and especially adjacent to every pin that requires RF ground. I found this may improve mixer balance and even IIP3 depending on frequency.

Strays

Above — For those who have trouble visualizing mixer math, this frequency domain sweep tells the story of mixing 2 frequencies in a diode ring.

This spring/summer I watched yet another YouTube video stating that mixing the RF + LO results in 2 frequencies. I wish it were true! In general, we can describe mixer output frequencies with the equation:

Tuesday, 14 April 2015

I spent some grand time on the bench this Fall and Winter. On May 3, I'll cease bench experiments + blog posting and go outside in our garden. To keep in touch with other radio and QRP work bench enthusiasts over the spring + summer, I'll employ Twitter.

Although, its not looking good time-wise, I hope to finish my Jupiter receiver and post a few of the 'unique circuits'. Reaching for something fresh, I'm avoiding cliche circuitry to learn and enjoy more.

Here's a quick glimpse at the front end filter — amplifier in test mode. I swept for S21 and performed DC measures last weekend.

To do my part — I hope to post a few more of my regen ideas/experiments over the Spring and Summer. However, please post your experiments and enjoyment of those simple to moderately complex regen radio sets that hold us in thrall.

Dave, AA7EE wrote about this budding new community in a nice post: Click here.

I'll end with some random photos. Thanks for reading. 73!

Above — I also design and make solid state guitar amplifiers --- although, I've never posted any. 1 day perhaps?

Above — Egad — I still have some tube stuff hanging around but may purge it out.

Friday, 3 April 2015

In winter 2015, I built 5 HF regens plus 2 VHF super regenerative receivers. I’ve run out of my better quality air variable capacitors, potentiometers and room — each version seems larger and uglier than the previous. With all these experiments, I’ve advanced about 2 mm up the regen receiver design learning curve — and in standard form factor, more questions arose than answers.

Briefly/frankly: I’m more a science officer Spock type than a green smoothie, quinoa and tofu devouring newager — thus I prefer to avoid emotional messaging and hyperbole. I share my experiments to kindle interest, invoke dialog and sincerely hope we’ll all improve and enjoy what’s left of the SWL bands + analog radio design. Out of the gate — I think of this radio as OK; enjoyed making it and wish you well with your own experiments.

Above — Photo of Regen #5 Front Panel.

1. Schematics

Above — Regen #5 RF Board Schematic

Above — Regen #5 AF Board Schematic

2. RF Preamplifier

A common gate amplifier provides reverse isolation. IMO, batteries are best suited for moors and trail. As a dedicated AC power supply enthusiast , I won’t run a regenerative receiver without this isolation to prevent my leaked signal getting 60 cycle modulated and coming back into the receiver antenna port (the cursed common mode hum thang), or perhaps, making the local Hams irate with my QRM.

Thanks to stuff like marijuana grow ops, our neighbor’s switch-mode lights, dimmers and other dumb dumbs; low band reception proves vexing in many bigger cities. I don’t want anything I’ve built adding to the radio listener interference burden.

I agree with the conclusions of regen wizard Charles Kitchin — the preamplifier should run at least 2-2.5 mA source (or emitter) current to help it avoid rectifying strong local stations. Lay the JFET flat side down on the copper board and solder the gate lead as close to the JFET plastic body as possible to squash UHF parasitic oscillations from that lead’s inductance. Further, the drain ferrite bead shown could just as easily be a low-value resistor. For example, 22 to 51 Ω.

Above — Close up of RF preamp circuitry and Q-multiplier tank inductor. I wound the 3.34 µH inductor with 22 gauge enamel coated wire on a T68-6 toroid.

3. Q-Multiplier

I sought a low distortion, high Q, negative resistance oscillator as the heart of this receiver and came up with this fun Colpitt’s variant. You can spend years learning, building and testing a ton of oscillator topologies and I plan to work towards this over time.

The regen control changes the oscillator amplitude. 1 annoyance with a Colpitts — as you adjust the amplitude or “regen” control potentiometer, the oscillator frequency changes since bias changes affects the transistor input capacitance (mainly through collector to base inter-element capacitance at the pn-junction).

Above — A diagram showing the various internal and parasitic capacitances that affect a transistor circuit.

To reduce the tank tuning effects from bias change, I employed 3 strategies that worked:

All my other BJTs above the fT of the PN5179 and BF199 are SMT parts. I wanted the RF board to only house leaded parts soldered in 100% Classic Ugly Construction plus — no low-Q, unknown temperature coefficient cut or glued pads anywhere on the RF board. Actually, leadless SMT parts probably offer the better BJT choice with respect to wiring parasitics.
It also might be better to run MPSH10 (PNP), BF199, PN5179 or other transistors with an fT between ~1- 2.5 GHz compared to my mega fT choice to reduce the chance of spurious oscillations while still fronting a low input C — I’ll leave transistor choice up to you.

The cascode configuration boosts the main Colpitts BJT’s output resistance to present a higher QL to the resonator. Better quality oscillators often run higher QL to isolate the tank from transistor variations and/or to reduce phase noise and possibly some temperature (frequency) drift.

4. Detector

While factoring each particular JFET’s characteristics, the physics behind this detector at various signal levels and Q-multiplier amplitude settings looms complex. I’ll just give a hypothesis for some points.

The detector is directly connected to the Q-M tank. To decouple it, I center tapped the main inductor.

A review of regenerative receiver operation

AM

For maximum AM sensitivity on a signal, advance the Q-M amplitude pot until you hear some high pitched AF noise or oscillation. Then slowly lower the Q-M amplitude just enough to eliminate this audio buzz. Instead, for weak signal listening, we may choose to autodyne the signal by further increasing the Q-M amplitude while adjusting the fine tuning capacitor to zero beat.

SSB/CW

For SSB, advance the Q-M amplitude pot until you hear some hiss and then go look for the familiar duck quack of a SSB signal. Fine tune around the signal and tweak the Q-M pot until you hear your desired audio quality. CW is straight forward — just find a nice beat frequency. If you run the RF gain too high, signals may get phase or frequency modulated.

Of course, for AM, or SSB/CW, a delicate interplay exists between the various controls so that you'll often dial in the best sounding audio by expert knob tweaking — the so-called, “art of regen” stuff that conjures mystique and nostalgia. This takes practice — regen receivers sure engage you!

Regen #5Particulars

Further muddying the waters, I ran a front panel switch that the places an 8K2 resistor in parallel with the 22K detector source resistor to make a "higher current" setting. In my particular case, that’s ~5974 Ω which sets the JFET source current to 441µA. I’ve probably mislabeled this switch on the receiver’s front panel: it might be better to just say lower current and higher current mode than SSB/AM.

When switched to the lower current setting, the measured source current = 138 µA.

AM

The gate-to-channel diode provides detection and the JFET is working as a square-law detector. Thus demodulation distortion will be a function of signal level and bias on the JFET since the square-law operation is only good over a certain range. The JFET is biased near to cutoff.

Normally, I run Regen#5 with the switch in high current mode for it offers maximum sensitivity. In certain cases such as when tuning strong AM signals, switching to the low current detector mode offers less AF distortion. Often, the switches' effects on recovered audio distortion sounds subtle.

I determined the 22K source resistor experimentally during listening tests. The goal = to find a source resistor that gives the best audio fidelity.

CW/ SSB

Ideally, we want our detector to operate as a mixer — in this case, we’re running a direct conversion receiver. In SSB/CW reception, you run greater Q-multiplier signal amplitude than while detecting AM signals and we’re probably get some square law detection within the Q multiplier.

1 theory is that rectification results in DC that may be enough to actually drive the FET toward pinch off which kills the gain. Switching in higher JFET source current will help keep the RF detected from the Q multiplier from pinching the JFET off.

This winter, I built some different, very low distortion AM detectors. In 1 design, I ran a low current pair of BJTs with heavy feedback. While remarkable for AM, they really sucked for CW/SSB detection. So the direct-coupled hybrid cascode detector shown is a compromise circuit that works OK for both but better for SSB than AM in terms of AF distortion via listening tests.

This detector begs for further experiments. For example, what happens when it is DC coupled to the Q-M tank and a high ohm gate resistor is added? Should the 8K2 shunt resistor be replaced with a pot, or perhaps a switch to allow 3 or more different JFET source currents?

Preamplifier

Nothing special here. A 0.1 µF input bypass rolls off noise and prevents local AM signals from entering, getting rectified and amplified by the AF chain. I opted to not include a multiple pole low-pass filter chain for dissecting CW/SSB pile ups, or a tone control circuit, but in a keeper-grade radio, I would add 1 or both.

I don’t run band-pass AF filtering in my receivers, but that's another option. With all the free, online software, designing good AF filters has never been easier.
You may boost the gain of the op-amps by increasing the feedback resistor value. Too much gain on strong signals may cause a spasm of feedback and create celestial noises.

PA

I’ve discussed this circuit before on this blog post.
Except now, I’ve solved a distortion problem caused by closing the loop in the op-amp connected to the power followers. In high gain feedback amplifiers, it does not take much time delay or phase lag to cause oscillation at high frequencies near the upper end of the bandwidth. The small 22 pF feedback capacitor lowers the closed loop bandwidth so that there is insufficient gain at high frequencies for oscillation to occur.

Above — FFT of the audio amplifier board. Swinging 10 Vpp with the strongest harmonic at - 57 dBc between a 12.2 to 0 VDC single-supply rail provided 1 of the happiest accomplishments in my fledgling amateur radio designer career. I hope to better this 1 day, but that will prove difficult.

Above — AF and RF circuit boards mounted and wired in Regen #5

5. Thoughts

I’ve seen countless regen schematics spawned by the good and the great. Wow! — how can such a relatively simple concept garner so much attention and appear in so many forms? I hardly feel I’ve contributed to the regen knowledge base, but gained valuable knowledge of what I want or hope to achieve in the future.

Regen#5 behaves like any regenerative radio should. The Q-M amplitude control feels precise — almost to the point of being too touchy. But does the regen amplitude control work any better or worse than other designs? I don’t know. The frequency shift associated with changing the Q-multiplier bias is minimal when compared to my other builds.

I applied standard LC VFO temperature stability techniques and felt surprised when I didn't have to add negative or positive tempco caps to stabilize it. This receiver stays on frequency for hours even when tuning CW and SSB signals.

Regen#5 suffers from microphonics as a SSB/CW receiver — and reminds me of an unbalanced or single-balanced DC receiver without the broadcast band nor in-band AM detection. The scratchiness of my dust-laden, ancient, air variable caps gets multiplied greatly by the sensitive RF chain. I’ll get some Caig Lab’s contact cleaner and have a go on those caps.

Still, too, the AM and SSB detector poses compromise — maybe it’s better to run 2 separate optimized detectors with a switch to pass the signal through the best detector for the desired demodulation mode? I’ve got lots of experiments ahead, and more regen circuit ideas to share but plan to stop all regen work until next fall.

6. Out Takes and Sound Bytes

Above — My prototype PA board

Above — FFT of the prototype PA board.

Above — Hartley VFO ideas from Regen #3

Above — FFT of the signal from the above Hartley.

Sound Bytes

I heavily compressed some sound bytes to show the receiver in real world conditions. You'll hear me tweaking the Q-multiplier control on these recordings: